April 2011 - Haskell-Cafe

Re: [Haskell-cafe] Assimp FFI Library
by John Lato 12 Apr '11

12 Apr '11

> > From: Joel Burget <joelburget(a)gmail.com> > Subject: [Haskell-cafe] Assimp FFI Library > To: haskell-cafe(a)haskell.org > Message-ID: <BANLkTimW-71NcBRqnr68kRAvKZytE1uVwA(a)mail.gmail.com> > Content-Type: text/plain; charset="iso-8859-1" > 5. I've reduced a lot of boilerplate in Vec.hs by using the CPP > preprocessor > extension. I could reduce the boilerplate by another factor of 3 if I could > recursively call templates but that's not allowed. I would like to have one > template to generate both of these lines: > > > data Vec N2 Double t = Vec2D !Double !Double deriving (Show, Eq) > > data Vec N3 Double t = Vec3D !Double !Double !Double deriving (Show, Eq) > > Notice there is an extra !Double in the second. Is there an easy way to do > this? I don't know much about Template Haskell, would that work? Would it > be > easy? > Yes, it works, and it's pretty easy. My adaptive-tuple package, http://hackage.haskell.org/package/adaptive-tuple, provides this (as well as strict vectors up to 20 elements). Tuple is a bit of a misnomer. You'll need to download the source to get the Template Haskell splices because they aren't exported (tarball from hackage, or via http://www.tiresiaspress.us/haskell/adaptive-tuple/) John L.

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Call for Contributions - Haskell Communities and Activities Report, May 2011 edition
by Janis Voigtländer 12 Apr '11

12 Apr '11

Dear all, I would like to collect contributions for the 20th edition of the ================================================================ Haskell Communities & Activities Report http://www.haskell.org/haskellwiki/Haskell_Communities_and_Activities_Report Submission deadline: 1 May 2011 (please send your contributions to hcar at haskell.org, in plain text or LaTeX format) ================================================================ This is the short story: * If you are working on any project that is in some way related to Haskell, please write a short entry and submit it. Even if the project is very small or unfinished or you think it is not important enough -- please reconsider and submit an entry anyway! * If you are interested in an existing project related to Haskell that has not previously been mentioned in the HCAR, please tell me, so that I can contact the project leaders and ask them to submit an entry. * Feel free to pass on this call for contributions to others that might be interested. More detailed information: The Haskell Communities & Activities Report is a bi-annual overview of the state of Haskell as well as Haskell-related projects over the last, and possibly the upcoming six months. If you have only recently been exposed to Haskell, it might be a good idea to browse the November 2010 edition -- you will find interesting topics described as well as several starting points and links that may provide answers to many questions. Contributions will be collected until the submission deadline. They will then be compiled into a coherent report that is published online as soon as it is ready. As always, this is a great opportunity to update your webpages, make new releases, announce or even start new projects, or to talk about developments you want every Haskeller to know about! Looking forward to your contributions, Janis (current editor) FAQ: Q: What format should I write in? A: The required format is a LaTeX source file, adhering to the template that is available at: http://haskell.org/communities/05-2011/template.tex There is also a LaTeX style file at http://haskell.org/communities/05-2011/hcar.sty that you can use to preview your entry. If you do not know LaTeX, then use plain text. If you modify an old entry that you have written for an earlier edition of the report, you should already have received your old entry as a template (provided I have your valid email address). Please modify that template, rather than using your own version of the old entry as a template. Q: Can I include Haskell code? A: Yes. Please consider using lhs2tex syntax (http://people.cs.uu.nl/andres/lhs2tex/) The report is compiled in mode polycode.fmt. Q: Can I include images? A: Yes, you are even encouraged to do so. Please use .jpg format, then. Q: Should I send files in .zip archives or similar? A: No, plain file attachements are the way. If you have several entries to submit, it helps if you send them in separate emails. Q: How much should I write? A: Authors are asked to limit entries to about one column of text. This corresponds to approximately one page, or 40 lines of text, with the above style and template. A general introduction is helpful. Apart from that, you should focus on recent or upcoming developments. Pointers to online content can be given for more comprehensive or "historic" overviews of a project. Images do not count towards the length limit, so you may want to use this opportunity to pep up entries. There is no minimum length of an entry! The report aims for being as complete as possible, so please consider writing an entry, even if it is only a few lines long. Q: Which topics are relevant? A: All topics which are related to Haskell in some way are relevant. We usually had reports from users of Haskell (private, academic, or commercial), from authors or contributors to projects related to Haskell, from people working on the Haskell language, libraries, on language extensions or variants. We also like reports about distributions of Haskell software, Haskell infrastructure, books and tutorials on Haskell. Reports on past and upcoming events related to Haskell are also relevant. Finally, there might be new topics we do not even think about. As a rule of thumb: if in doubt, then it probably is relevant and has a place in the HCAR. You can also ask the editor. Q: Is unfinished work relevant? Are ideas for projects relevant? A: Yes! You can use the HCAR to talk about projects you are currently working on. You can use it to look for other developers that might help you. You can use it to write "wishlist" items for libraries and language features you would like to see implemented. Q: If I do not update my entry, but want to keep it in the report, what should I do? A: Tell the editor that there are no changes. The old entry will typically be reused in this case, but it might be dropped if it is older than a year, to give more room and more attention to projects that change a lot. Do not resend complete entries if you have not changed them.

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Assimp FFI Library
by Joel Burget 12 Apr '11

12 Apr '11

Hello, I've been working on an ffi library for the Assimp asset import library( http://assimp.sourceforge.net) It should be useful for people doing graphics in Haskell. I've been working on it so I can import models into a ray-tracer I've been working on. My current progress is here: https://github.com/joelburget/assimp. A couple weeks ago I managed to import models and do some simple (and slightly buggy) renders, but since then I've been redoing a lot of the guts of the library. At this moment it doesn't build because of the changes I'm in the middle of. Hopefully I'll be able to release the first version in the next week or so. I've run into a few roadblocks that I would be thankful for help with. 1. In Graphics.Formats.Assimp.Vec I'm trying to get the fastest and smallest possible vector types since a program could easily have millions of vectors in memory and operations on them take a huge fraction of the time in an average program. One problem I ran into is how to make the vectors type-safe. The assimp library has separate data types for colors and vectors, but represented the same in memory. I have a few ideas about how to represent these: a. The way I've been doing it: >data N2 >data N3 >data N4 >data Color >class Num a => Vector n a where > data Vec n a :: * -> * > ... rest of class >-- A 3-dimensional vector of doubles. t is the (phantom) tag type, like Color, Direction, etc >-- This is meant to improve type safety >instance Vector N3 Double where > data Vec N3 Double t = Vec3D !Double !Double !Double deriving (Show, Eq) > ... rest of instance This seems very close to what I want, but it presents a few problems. First, I create matrices from vectors, so the matrix must have an implicit type, the type of the tags from the vectors it is made of. I made this () by convention but then to multiply a vector by this matrix I have to use the dot product and other vector operations, but then the tag type from the matrix is different from that of the matrix I am multiplying. This leads to the type `dot :: Vec n a t1 -> Vec n a t2 -> a`. Notice how there are two different tag types, reducing type safety. Another problem is importing vectors from the library. I would like to have different tags for distance and direction, again to increase type safety, but they are both represented the same way in the library, so the Storable instance can't always know what type to create. I would probably always have to return a vector with tag type (). b. Just create separate Vec and Color datatypes. I would be mirroring the library directly. Both would be instances of Vector. c. Use newtype wrappers around Vec to represent the different types. I don't really like this solution because the newtypes are too painful to use. d. I've thought about some other solutions that I've forgotten now. Maybe you have something better! Remember, I would like to have a high degree of type safety if possible, but the top consideration is performance. Another thing I've been wondering about: could I use overlapping instances to allow the vectors to be of any numeric type, but specialize and unbox them for the common ones? I'm assuming unboxing can't be done unless we have a specific type because the compiler won't know how large the type is. 2. How do people feel about the vector and matrix operators (eg |*|, |*||, ||*)? I like them a lot but I would like to see how other people feel about them. 3. Do I have to worry about marshalling float, double, and int straight to Haskell? The report says Float and Double should conform to the IEEE standard. I'm more interested in Int since the report only requires a 30 bit Int. On my computer they appear to be 64 bits. Does this vary on 32 bit computers? 4. There are several places the original library uses unsigned ints. Obviously they can't be negative so I used CUInt in the Haskell code. I never see CUInt instead of Int in real code. Should I change these? 5. I've reduced a lot of boilerplate in Vec.hs by using the CPP preprocessor extension. I could reduce the boilerplate by another factor of 3 if I could recursively call templates but that's not allowed. I would like to have one template to generate both of these lines: > data Vec N2 Double t = Vec2D !Double !Double deriving (Show, Eq) > data Vec N3 Double t = Vec3D !Double !Double !Double deriving (Show, Eq) Notice there is an extra !Double in the second. Is there an easy way to do this? I don't know much about Template Haskell, would that work? Would it be easy? I should mention that I'm going to convert all the Storable instances from something like this: > peek p = do > w <- (#peek aiQuaternion, w) p > x <- (#peek aiQuaternion, x) p > y <- (#peek aiQuaternion, y) p > z <- (#peek aiQuaternion, z) p > return $ Quaternion w x y z to something like this: > peek p = Quaternion <$> (#peek aiQuaternion, w) p > <*> (#peek aiQuaternion, w) p > <*> (#peek aiQuaternion, w) p > <*> (#peek aiQuaternion, w) p So there's no need to tell me to change those. But please let me know of other places that are in poor style. Oh yes, patches and collaborators are welcome. Thanks! Joel

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Stateful iteratees
by Ertugrul Soeylemez 12 Apr '11

12 Apr '11

Hello fellow Haskellers, I'm trying to solve a very practical problem: I need a stateful iteratee monad transformer. Explicit state passing is very inconvenient and would destroy the elegance of my library. There are two approaches to this: 1. type MyT a m = Iteratee a (StateT MyConfig m) 2. type MyT a m = StateT MyConfig (Iteratee a m) Both work well except in two very specific corner cases: - I need to convert the transformer to 'Iteratee a m', i.e. remove the state layer. This is obviously trivial with the second variant, but seems very difficult with the first one, if it's possible at all. - I need to use control structures of Iteratee like catchError. This is obviously trivial with the first variant, but very inconvenient with the second, because I would need to reinvent many wheels. Does someone know a cleaner, more elegant solution? Encapsulating the state in the iteratee's input type is not an option. Many thanks in advance. Greets, Ertugrul -- nightmare = unsafePerformIO (getWrongWife >>= sex) http://ertes.de/

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Re: [Haskell-cafe] Foreign function performance: monadic vs pure
by John Lato 12 Apr '11

12 Apr '11

> > From: Gregory Collins <greg(a)gregorycollins.net> > > On Mon, Apr 11, 2011 at 3:55 PM, Serguei Son <serguei.son(a)gmail.com> > wrote: > > So if I must use a safe function returning IO a, > > there is no way to improve its performance? To give you > > a benchmark, calling gsl_ran_ugaussian a million times > > in pure C takes only a second or two on my system. > > In the C version, are you also producing a linked list containing all > of the values? Because that's what mapM does. Your test is mostly > measuring the cost of allocating and filling ~3 million machine words > on the heap. Try mapM_ instead. > I've done some rough tests of a few versions, and I believe Gregory's analysis is correct. These are for calculating a sum of sines of 1..10^7 with ghc-7.0.3. version execution time IO, safe - 4.08s Pure, safe - 3.07s IO, unsafe - 2.77s Pure, unsafe - 1.95s For the IO versions, I used the following: main = foldM (\ !acc n -> fmap (acc +) $ c_sin_io n) [1..10^7] >>= print (this can possibly be improved; it was the first thing I thought of) The most important factor (as often the case with Haskell) is to pay attention to the space properties of your algorithm. Instead of using mapM, which forces the spine of the list, it's much better to interleave the fold step with the other IO. Note that the "acc" parameter is strict, this is necessary to avoid building a thunk. John L.

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Tuple
by Anwar Bari 12 Apr '11

12 Apr '11

HI Cafe I have to make a function to check that I have one occurrence of the last element (z) of the same list [a,b] in the tuple [([a,b],z)] For example [([1,2],3),([1,1],5),([1,3],6).......] this is true because there is one single z for each single list. while this one is false [([1,2],3),([1,2],5),([1,3],6).......] because 3&5 were found for the same list [1,2] any Idea how to code this Fn. Thanks

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Stacking data types
by Yves Parès 11 Apr '11

11 Apr '11

Hello Café, I'm trying to get some modular data types. The idea that came to me is that I could stack them, for instance : data Character a = Character { life :: Int, charaInner :: a } data Gun a = Gun { firepower :: Int, gunInner :: a } data Armor a = Armor { resistance :: Int, armorInner :: a } Then a character with a gun and an armor can be build this way: chara = Character 100 $ Armor 40 $ Gun 12 The idea now is to be able to get some part of the character: itsGun :: Character ?? -> Gun ?? itsGun = content Then content would be a class method: class Has b a where content :: a -> b And it would be recursively defined so that: instance (Has c b, Has b a) => Has c a where content = (content :: b -> c) . (content :: a -> b) Then itsGun would be more like: itsGun :: (Has Gun a) => a -> Gun ?? itsGun = content But after some juggling with extensions (ScopedTypeVariables, UndecidableInstances, IncoherentInstances...) I can't get it working. Has someone a simpler way to achieve modular types?

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Foreign function performance: monadic vs pure
by Serguei Son 11 Apr '11

11 Apr '11

Consider two versions of sin wrapped: foreign import ccall "math.h sin" c_sin_m :: CDouble -> IO CDouble and foreign import ccall "math.h sin" c_sin :: CDouble -> CDouble One can invoke them so: mapM c_sin_m [1..n] mapM (return . c_sin) [1..n] On my computer with n = 10^7 the first version never finishes, whereas the second one calculates the result within seconds. To give you my context, I need to call a random variable generator multiple times, so that it must return IO a. Any explanation for this behavior?

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Emscripten: compiling LLVM to JavaScript
by Sönke Hahn 11 Apr '11

11 Apr '11

Hi all! I haven't tried the tool myself, but it seems interesting to the Haskell efforts to compile to JavaScript: http://syntensity.blogspot.com/2011/04/emscripten-10.html Cheers, Sönke

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Re: [Haskell-cafe] already installed packages alerted as not being installed
by Kazu Yamamoto 11 Apr '11

11 Apr '11

Hello, > thanks I wanted to mention that the "unknown symbol" error is very > likely not related to the cab tool as the same error appears, when > using the cabal - tool. I guess we can ignore it even in the context > of my main question, sorry for being to verbose. What I found more > interesting is, that cab doesn't try to re-install an installed > package - already in the database too, as ghc-pkg output pretends - > which is expected behaviour. The cabal system frontend obviously does > trying to reinstall the package though. This seems like different > model of reality... This is not true. cab install re-installs some packages if necessary, unfortunately. That's why I recommend to use "cab install -n". --Kazu

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